Images of the interiors of bodies may be acquired using various types of tomographic techniques, which involve recording and measuring radiation from tissues and processing acquired data into images.
One of these tomographic techniques is positron emission tomography (PET), which involves determining spatial distribution of a selected substance throughout the body and facilitates detection of changes in the concentration of that substance over time, thus allowing to determine the metabolic rates in tissue cells.
The selected substance is a radiopharmaceutical administered to the examined object (e.g. a patient) before the PET scan. The radiopharmaceutical, also referred to as an isotopic tracer, is a chemical substance having at least one atom replaced by a radioactive isotope, e.g. 11C, 15O, 13N, 18F, selected so that it undergoes radioactive decay including the emission of a positron (antielectron). The positron is emitted from the atom nucleus and penetrates into the object's tissue, where it is annihilated in reaction with an electron present within the object's body.
The phenomenon of positron and electron annihilation, constituting the principle of PET imaging, consists in converting the masses of both particles into energy emitted as annihilation photons, each having the energy of 511 keV. A single annihilation event usually leads to formation of two photons that diverge in opposite directions at the angle of 180° in accordance with the law of conservation of the momentum within the electron-positron pair's rest frame, with the straight line of photon emission being referred to as the line of response (LOR). The stream of photons generated in the above process is referred to as gamma radiation and each photon is referred to as gamma quantum to highlight the nuclear origin of this radiation. The gamma quanta are capable of penetrating matter, including tissues of living organisms, facilitating their detection at certain distance from object's body. The process of annihilation of the positron-electron pair usually occurs at a distance of several millimetres from the place of the radioactive decay of the isotopic tracer. This distance constitutes a natural limitation of the spatial resolution of PET images to a few millimetres.
A PET scanner comprises detection devices used to detect gamma radiation as well as electronic hardware and software allowing to determine the position of the positron-electron pair annihilation on the basis of the position and time of detection of a particular pair of the gamma quanta. The radiation detectors are usually arranged in layers forming a ring around object's body and are mainly made of an inorganic scintillation material. A gamma quantum enters the scintillator, which absorbs its energy to re-emit it in the form of light (a stream of photons). The mechanism of gamma quantum energy absorption within the scintillator may be of dual nature, occurring either by means of the Compton's effect or by means of the photoelectric phenomenon, with only the photoelectric phenomenon being taken into account in calculations carried out by current PET scanners. Thus, it is assumed that the number of photons generated in the scintillator material is proportional to the energy of gamma quanta deposited within the scintillator.
When two annihilation gamma quanta are detected by a pair of detectors at a time interval not larger than several nanoseconds, i.e. in coincidence, the position of annihilation position along the line of response may be determined, i.e. along the line connecting the detector centres or the positions within the scintillator strips where the energy of the gamma quanta was deposited. The coordinates of annihilation place are obtained from the difference in times of arrival of two gamma quanta to the detectors located at both ends of the LOR. In the prior art literature, this technique is referred to as the time of flight (TOF) technique, and the PET scanners utilizing time measurements are referred to as TOF-PET scanners. This technique requires that the scintillator has time resolution of a few hundred picoseconds.
Another method of imaging is MRI (Magnetic Resonance Imaging), which uses the magnetic properties of atomic nuclei, in particular, nuclei of hydrogen atoms, that is protons widely occurring in matter, including tissues of living organisms. The MRI technique allows obtaining images of the density distribution of hydrogen atoms giving the morphological image of tissues.
Superimposing of a functional image (PET) over a morphological image (MRI) considerably increases the capabilities of imaging techniques: a PET image enables precise positioning of metabolic changes in individual organs and the determination of the degree of these changes, whereas the obtainment of an MRI image at the same time allows a precise allocation of these changes to respective organs. Obtained hybrid PET/MRI images may be useful in scientific research on physiological processes, where it is especially important to precisely assign to respective tissues metabolic changes of tested radiopharmaceuticals, during imaging.
Today, in many laboratories in the world, technology that would allow for simultaneous PET and MRI imaging is intensively developed. Known PET/MRI hybrid tomographs are devices in which the PET tomograph and the MRI tomograph are spatially separated. The main difficulty in combining the two imaging techniques is due to mutual interruption of signals between PET and MRI detection systems. Strong magnetic fields used in MRI interfere with operation of converters of light impulses into electrical impulses as well as they disturb transmission and processing of the signals in PET detectors. Such design of a device causes that PET and MRI imaging is, in fact, carried out in different places of object's body and at different time—the object is moved incrementally between successive imaging, thus it is required to move the object and to stop him between successive imaging. This procedure involves a threat that image distortions, so-called artefacts, may occur, especially in abdominal cavity organs, which may move between individual scanning events due to accelerations to which the object is subjected during shifting. Moreover, the superimposing of MRI and PET images, taken at different times, over each other, requires that additional corrections should be introduced due to the weakening activity of the radiopharmaceutical and metabolic processes; what also needs to be remembered is that each of these corrections is additionally exposed to systemic errors that occur when the images are superimposed. In turn, inserting a PET tomograph between coils of MRI tomograph and the object distorts the magnetic fields and reading of electromagnetic signals of the MRI tomograph due to eddy currents and electromagnetic waves induced in the electronics components used for reading and transmission of electrical signals of PET tomograph.
The state of the art technology tried to overcome the above mentioned problems and it describes equipment enabling simultaneous PET and MRI diagnostics.
A U.S. Pat. No. 8,013,607 discloses a solution wherein PET and MRI tomographs are spatially separated and aligned in close proximity to each other. The device allows sequential PET and MRI scans and the object, during the examination, is placed on the platform and moved between the tomographs. A similar solution was also described in the article “Design and performance evaluation of a whole-body Ingenuity TF PET/MRI system” (Z. Zaidi et al. Phys Med. Biol. 56 (2011), pp. 3091-3106). The disclosed technique avoids the technical difficulties related to the negative impact of PET detectors on magnetic fields and MRI electromagnetic signals through the physical separation of the two detectors. However, moving the object between individual imaging can lead to distortion in superimposed PET and MRI images (so-called artefacts), especially in the case of abdominal organs, which can move between the individual scanning activities as a result of acceleration experienced by the object when moving.
The article “Simultaneous PET and NMR” (P K Marsden et al. Brit J Radiology 75 (2002) pp. 53-59), describes a hybrid tomograph with non-standard readout by carrying signals over long optical fibres, which are inserted inside the MRI scanner. However, the use of this solution reduces the imaging field of view and PET imaging quality deteriorates due to the need for signals to be transmitted in several-metre thin optical fibres.
The article “Whole-Body MR/PET Hybrid Imaging: Technical Considerations, Clinical Workflow, and Initial Results” (Quick H. et al., MAGNETOM Flash January/2011 pp. 88-100) presents the possibility of using silicon photomultipliers or avalanche diodes instead of the standard photomultiplier tubes, and enclosing them along with electronics in an electromagnetic housing made, for example, of copper and inserting them between the gradient coil and the signal-readout coil of MRI tomograph. A similar solution consisting in using silicon photomultipliers is also disclosed in the patent description U.S. Pat. No. 7,218,112. The described method allows simultaneous imaging in a relatively large transverse field of view. This solution is schematically illustrated in FIG. 1, in which the PET 20 detectors are located between the receiving-transmitting coils 31 surrounding the object 5 and the gradient coils 32. PET detectors are made of LSO crystals 21 with an avalanche photodiode matrix 22 integrated with a cooling system 23 and analogue readout electronics 24. Detection modules have shields made of copper. Such a layout of PET and MRI tomograph elements can, however, lead to distortions of magnetic fields and electromagnetic signals used in MRI and distortions of signals in PET tomograph. The main factors causing the disorders described above are: (i) converters, electronics and cooling systems, which are, as per the solution, between the receiving-transmitting coils and gradient coils, (ii) transmission of electrical signals from PET detectors between the receiving-transmitting coils and gradient coils, (iii) scattering of annihilation quanta in the receiving-transmitting MRI coils located between the object and the layer of PET detectors. Furthermore, the presented solution is expensive, and the cost of the detector and electronics increases approximately linearly with the length of the longitudinal field of view, which is a significant limitation preventing large-scale production of hybrid PET/MRI tomographs with a large longitudinal field of view.
A US patent application US20120112079 describes a strip device and the method used in the determination of position and time of gamma quanta reaction, and the application of this device in PET. The TOF-PET tomograph, described in the application, allows simultaneous imaging of the whole object's body, while the material used to register gamma quanta is polymers doped with elements of high atomic numbers. The device described in this application reduces the cost of PET tomography. US20120112079 does not present, however, a method for simultaneous PET and MRI imaging using polymer scintillator strips.
A PCT application WO2006119085 discloses an integrated PET-MRI scanner. This integrated scanner includes a main magnet that generates a magnetic field, wherein images of the subject is generated in a central region of the magnetic field. It also includes a PET scanner which is enclosed by the main magnet. The PET scanner further comprises: at least one ring of scintillators, which is situated in the central region of the magnetic field and, one or more photodetectors, which are coupled to the ring of scintillators, so that the one or more photodetectors are outside the central region of the magnetic field. The integrated scanner also includes radio frequency (RF) coils which are enclosed by the PET scanner. By keeping the photodetectors and associated circuitry outside the central region of the magnetic field, the integrated scanner reduces the electromagnetic interference (EMI) between the PET scanner and the MRI scanner. The gamma scintillators are positioned only in the central region of the magnetic field and the photoelectric converters are positioned in the working area of the MRI scanner. The scintillators are made from crystals: LSO, BGO.
It would be desirable to provide an imaging device utilizing polymer scintillators, which would enable simultaneous registration of gamma radiation and execution of nuclear magnetic resonance with a large field of view, enabling the elimination of any artefacts that could distort the image due to the movement of the object, and systematic errors formed during superimposure of images made at various positions and times. This will allow effective, simultaneous functional and morphological imaging.